Although gasoline engines were initially operated in ways such that particulates were not formed, gasoline direct injection (GDI) technologies were later introduced which involve conditions of stratified combustion resulting in lean burn conditions and improved fuel efficiency. Such conditions, however, can lead to the generation of particulates. Particulate emissions for direct injection engines are being subject to regulations including the upcoming Euro 5 (September 2009) and 6 (September 2014) standards. Existing aftertreatment systems for gasoline engines are not suitable for achieving the proposed particulate matter standard. In contrast to particulates generated by diesel lean burning engines, the particulates generated by gasoline direct injection engines tend to be finer and in lesser quantities. This is due to the different combustion conditions of a diesel engine as compared to a gasoline engine. For example, gasoline engines run at a higher temperature than diesel engines. Thus exhaust gas from diesel engines display temperatures generally ranging from 250 to 500° C., whereas exhaust gas from gasoline engines usually have a temperature ranging from 800 to 900° C. Also, hydrocarbon components are different in the emissions of gasoline engines as compared to diesel engines.
Thus, due to the different composition and temperature of exhaust gas streams resulting from gasoline engines compared to diesel engines, in particular with respect to the far lower ratios of soot to hydrocarbon, carbon monoxide and nitrous oxide exhaust gas pollutants, respectively, a different treatment concept is necessary both regarding the type and composition of the apparatus involved in exhaust gas treatment such as particle filters, TWC, and NOx-traps, as well as the arrangement of these components in a system adapted for the treatment of such exhaust gas streams. More specifically, while diesel engine exhaust gas streams will typically contain about 0.14 wt.-% of total hydrocarbon, CO and NOx pollutants (i.e. about 1.2 g/km of hydrocarbons, about 0.3 g/km of CO, and about 0.23 g/km of NOx) for about 0.02-0.07 g/km of soot, gasoline engine exhaust gas typically contains about 1.1 wt.-% of total hydrocarbon, CO and NO pollutants (i.e. about 5.2 g/km of hydrocarbons, about 1.5 g/km of CO, and about 3.4 g/km of NOx) for about 0.0001-0.001 g/km of soot. Although exhaust gas from gasoline direct injection engines typically contain somewhat less hydrocarbon, CO and NO pollutants and slightly more soot (i.e. about 0.001-0.002 g/km), these proportions are still far from resembling diesel exhaust gas compositions. Further differences regarding particle size and particle size distribution of the soot particles in diesel and gasoline engine exhaust gas streams, as well as the different exhaust gas stream temperatures resulting from diesel and gasoline combustion in the respective engine types leads to completely different scenarios, such that diesel engine exhaust gas treatment technologies may not be readily applied to the technical field of gasoline engine exhaust gas treatment.
Besides regulations for the treatment of exhaust gas particles, emission standards for unburned hydrocarbons, carbon monoxide and nitrogen oxide contaminants also continue to become more stringent. In order to meet such standards, catalytic converters containing a three-way conversion (TWC) catalyst are located in the exhaust gas line of internal combustion engines. In particular, said catalyst promotes the oxidation by oxygen in the exhaust gas stream of unburned hydrocarbons and carbon monoxide as well as the reduction of nitrogen oxides to nitrogen.
With respect to treatment of diesel exhaust gases and particulates, the prior art generally provides for an oxidation catalyst upstream of a particulate filter. A clean-up catalyst downstream of an oxidation catalyst and particulate filter combination is provided in U.S. Patent Application No. 2007/0137187. Suitable clean-up catalysts downstream of the filter include a further oxidation catalyst or a TWC catalyst located on a substrate carrier such as a flow-through monolith.
Particulate filters used in diesel systems have been coated with, for example, soot burning catalysts that facilitate passive regeneration of soot. In addition, U.S. Pat. No. 7,229,597 provides a catalyzed selective catalytic reduction (SCR) filter downstream of an oxidation catalyst for simultaneous treatment of nitrogen oxides (NOx), particulate matter, and hydrocarbons. Further, U.S. Patent Application No. 2004/0219077 discloses a catalyzed filter in communication with a diesel engine exhaust. Placement of catalysts on soot filters, however, leads to gradual loss of effectiveness due to deleterious components of the diesel exhaust stream. A sufficient loading of catalyst is needed to achieve treatment goals, but this should be balanced with the need to provide acceptable back pressure in the system.
In addition to these, EP 2 042 226 A discloses a particulate filter for engines with a primarily stoichiometric regimen regarding the air:fuel ratio of the combustion mixture. In particular, said document teaches a layered catalyst design, wherein a layer containing Rh directly covers a layer containing Pd, and only the Rh-containing layer further comprises an oxygen-storage component (OSC).
It is therefore the object of the present invention to provide a treatment system for a gasoline engine exhaust stream as well as a method for the treatment of gasoline engine exhaust gas, in particular regarding the treatment of gasoline engine exhaust gas from gasoline direct injection engines.